Regenerative type periodic flow devices are conventionally employed for the transfer of heat or of other constituents from one fluid stream to another, and thereby from one area or zone in space to another. Typically, a sorptive mass is used to collect heat or a particular mass component from one fluid stream which flows over or through the sorptive mass. The flowing fluid is rendered either cooler (in the case of heat sorption) or less concentrated (in the case of, for instance, adsorption of particular gases). The sorptive mass is then taken "off-stream" and regenerated by exposure to a second fluid stream which is capable of accepting the heat or material desorbed with favorable energetics.
In many instances, the sorptive material is contained within a vessel or distributed within a bed structure. It is desirable that such material is provided with maximum surface area, and that fluid flows through the sorptive material matrix be smooth (non-turbulent) and regular. Once the sorptive material has been saturated (i.e. has reached its maximum designed capacity for sorption), the vessel or bed is then removed from the fluid flow path and exposed to a second fluid flow to regenerate the sorptive capacity of the material by, for instance, cooling the sorptive material or desorbing material taken up during "on-stream" operation. After such regeneration, the sorptive material is once more placed back "on-stream" and the operation continues.
From such single cycle systems evolved multiple vessel systems which permitted semi-continuous (or semi-batch) operation by synchronously alternating two or more sorptive vessels between on-stream and off-stream operation. The choice of numbers of vessels and cycle structures depends on many factors, but most importantly the ratio between consumption rate of the sorptive capacity of the vessel, and regeneration rates for that same vessel.
In some applications, semi-continuous systems have evolved into continuous flow systems where the sorptive media itself is moved between two or more flowing fluid streams. The most common construction employed for such systems is a porous disk, often referred to as a wheel or rotor. In its simplest form, such a wheel is divided into two flow zones, and fluid is passed over the sorptive surface of the wheel (typically flowing through the thickness of the disc parallel to the rotational axis of the cylinder) as the wheel is rotated to carry the sorptive material from one zone, into the other, and back again to complete a revolution. In a heat exchanger wheel, for instance, one zone of warm fluid and one zone of cooler fluid are present. Heat is adsorbed by the material of the wheel in the warm flow zone, and is carried away from the wheel as the sorptive material passes through the cool flow zone.
Typically wheel systems are designed according to predefined parameters including known fluid characteristics, known flow rates, known temperatures/concentrations, known and preselected sorptive characteristics (sorption constants and capacities), known wheel geometry, and preselected wheel rotational speeds. Although designed for a particular set of characteristic operating conditions, wheel system manufacturers typically provide information about operation at other conditions. This information is typically derived empirically for a given system and the relationships identified by such methods are valid only over very limited ranges of conditions. For a given system, there is no available means which permits optimization of performance (as either capacity or efficiency) over a wide range of operational conditions.
There have been attempts to employ closed-loop control systems to adjust the operation of wheel sorption systems to changing operating conditions. These prior art systems have been unsuccessful primarily due to the large time constants of the physical systems themselves. The time constant of such a system is a measure of the amount of time required for the system to achieve a steady state after a change in conditions or operating parameters. For example, for a typical air to air heat exchanger system, the time constant may be on the order of 75 seconds. However, for a desiccant/water vapor mass exchanger, the time constant may well exceed 75 minutes. In typical control systems which control operational parameters such as wheel rotational speed based on uncontrolled independent ambient conditions, response times tend to promote over control of the system and tend to destroy stability. For systems incorporating appropriate integration time constants, the ability of the system to react to changing conditions is so limited as to negate any effect of the control system on the efficiency of the system.